I've spent some time experimenting tonight. It seems that I hit a limit at 3 shades of gray for the placement of ecosystem objects. (By the way, thanks Rutra for the explanation about the cutoff filters. I'll see what I can do with that.) 

Does anyone know if you can alter the orientation of ecosystem elements with a bitmap so you can determine which direction houses face, for instance? 

Download my free stuff here: http://www.renderosity.com/homepage.php?page=2&userid=323368